"Don't see a border as a way of creating conflict, that will only cause real hardship."

Checkpoint Charlie in Berlin was the most famous crossing point between East and West Germany during the Cold War and came to symbolise divisions between the West and the Soviet Union.

Gibraltar is a tiny British overseas territory on the southern tip of Europe which voted overwhelmingly against Brexit.

It is not in the Customs Union and produces virtually nothing, importing its labour and goods.

It uses cameras and guards to police its short border with Spain, Mr Picardo said.

Around 13,000 people enter Gibraltar from Spain for work each morning. There are two crossings over a frontier just three quarters of a mile long, one for commercial and one for non-commercial traffic.

Coachloads of tourists are bussed in.

Mr Picardo said it would be impossible to put Heathrow-style machine readers in place to force everybody to scan their passports.

The chief minister added: "We must not allow anyone to use a border crossing as a political choke point. "

Spain has long contested Britain's 300 year-rule of Gibraltar, which has a population of about 30,000.

Earlier this year there was a row after draft Brexit negotiating guidelines published by the EU said Spain would be consulted on any decisions affecting Gibraltar.

Mr Picardo said: "It is one of the most fraught relationships in international diplomacy."

He said his territory was the second largest employer in Andalusia in southern Spain.

The chief minister pledged to visit Northern Ireland in the near future and is scheduled to meet DUP leader Arlene Foster and Sinn Fein's Michelle O'Neill as Brexit talks in Brussels intensify.

A leaked European Commission document suggested that the Republic is now pushing hard for concrete reassurance on the border question ahead of the crucial EU leaders' summit in December at which Theresa May hopes to gain a green light for trade talks to begin.

Along with the question of expat citizens' rights and the UK's "divorce bill", the Irish border is a key issue in the first stage of Brexit talks, on which "sufficient progress" must be made before the leaders of the remaining 27 EU states will give the go-ahead for trade talks.